Driving at below the speed which other drivers might reasonably expect to be able to drive (but while still within the speed limit) displays a poor standard of driving, as it leads to an increased number of overtaking manoeuvres and a possible build up of aggression among other drivers. It's not a specific offence but it would definitely get you marked down on a driving test (for 'failure to make progress').
However if the police (and subsequently the CPS) could show that other drivers were inconvenienced by your slow driving, it might result in a successful prosecution for driving "without reasonable consideration for other persons using the road". [Section 3, Road Traffic Act 1988]